Method for making a photomask with multiple absorption levels

ABSTRACT

A photomask for manufacturing a semiconductor device. The photomask is manufactured by a providing a photomask substrate and alternately depositing a plurality of layers of a light-absorbing material and of an etch-stop material on the photomask substrate. The light-absorbing material is selected as having a well-defined etching selectivity from that of the etch-stop material. The layers are successively patterned by removing by a selective etching process at least a portion of at least one of said layers, the portion removed from a lower, in relation to the substrate, layer a subset of the portion removed from a higher layer. Together, the patterned layers are used as a photomask to photolithographically imprint a pattern of a photoresist on a semiconductor wafer under manufacture. The photoresist is used in the etching process of the semiconductor wafer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the manufacture and use of aphotomask used in photolithographic processes during the manufacture ofintegrated circuits. More particularly, the present invention isdirected to a photomask offering controllable multipleradiation-absorption levels and to a method for manufacturing thephotomask.

2. Description of the Related Art

Certain integrated circuit fabrication processes require precisedelimitation of areas to be affected by the fabrication process and ofareas to be protected from the fabrication process. Photomasks arepatterned masks used in photolithographical processes for selectivelyinhibiting the exposure to radiation, such as light, specific regions ofa material to be patterned, while allowing radiation to act on otherregions.

Conventional photomasks include a patterned layer of a light-blockingmaterial, usually chromium, used to block transmission of the particularform of light used. Conventional chromium masks generally provide onlyone level of complete absorption--that is, light is either totallyblocked by the chromium or transmitted in those regions from which thechromium has been removed. These "on-off" photomasks are referred to asbinary intensity masks (BIM).

The exposure level of the photoresist, which is the material to bepatterned using the photomask, can be controlled by raising or loweringthe illumination level. This procedure causes all features defined bythe photomask to receive approximately the same light exposure. Toprovide greater versatility in the exposure of photoresist patterns,particularly in the submicron regime, it would be desirable to have somefeatures that receive relatively greater or less exposure than otherfeatures. For example, it is generally helpful if larger featuresreceive less exposure than small features. Of course, it is possible tovary the exposure levels by using separate photomasks and repeating theexposure process. However, this repetitive process is impractical,requiring additional masks and additional exposure and developmentsteps.

As an alternative to existing BIM photomasks, photomasks have beenprepared which function to phase-shift the light energy. The basicprinciple of such masks is to use a phase-shifting material to interferewith the electric fields of light passing through adjacent open featuresand, thus, cause the annihilation of superimposed fields with oppositephases. One type of phase-shifting photomask, an attenuatedphase-shifting mask (APSM), uses a film of a slightly transmissiveabsorber with a 180° phase shift. By controlling the thickness andoptical properties of the mask, such as by changing the chemical contentof the film, two levels of light-transmission can be achieved in asingle mask.

However, phase-shifting systems provide only limited control ofabsorption levels and require complex manufacturing processes.Additionally, researchers of phase-shifting systems have encountereddifficulties applying such systems to arbitrary mask patterns andobtaining accurate feature delineation.

In view of the problems described above, a need remains for a photomaskwhich can be efficiently manufactured and which offers multipleabsorption levels. Accordingly, there is provided herein a photomask andmethod of its use which allows different portions of the photoresist tobe exposed simultaneously to various levels of exposure using a singleexposure process.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a semiconductor manufacturer mask. The mask includes aplurality of radiation-absorbing layers. At least one etch-stop layer isdisposed between the radiation absorbing layers.

In accordance with another aspect of the present invention, there isprovided a semiconductor manufacturer mask having multiplelight-absorption levels. The mask includes a plurality oflight-absorbing layers. At least one etch-stop layer is disposed betweenthe light-absorbing layers.

In accordance with yet another aspect of the present invention, there isprovided a method for manufacturing a photomask having multiplelight-absorbing levels. A photomask substrate is provided. A pluralityof alternating layers of a light-absorbing material and of an etch-stopmaterial are formed on the photomask substrate. The light-absorbingmaterial and the etch-stop material are selectively etchable in relationto each other.

In accordance with still another aspect of the present invention, thereis provided a method for manufacturing a photomask having multiplelight-absorbing levels. A photomask substrate is provided. A first layerof a generally light-absorbing material is deposited onto the substrate.A second layer of a buffer material is formed onto the first layer. Thesecond layer of buffer material is selectively etchable relative to thefirst layer. A third layer of a generally light-absorbing material isdeposited. The third layer is selectively etchable relative to thesecond layer.

In accordance with a further aspect of the present invention, there isprovided a method for patterning a semiconductor device. A photomasksubstrate is provided. A plurality of alternating layers of alight-absorbing material and of an etch stop material is formed on thephotomask substrate. The layers are successfully patterned by removingat least a portion of at least one of the layers by selective etchingprocess to form a photomask. A semiconductor wafer is provided. Aphotoresist is applied to at least a portion of the surface of thesemiconductor wafer. The photomask is aligned over at least a portion ofthe photoresist. The photomask is exposed to light. The photoresist isdeveloped.

In accordance with a still further aspect of the present invention,there is provided a method for manufacturing a semiconductor device. Aphotomask is provided. The photomask includes the plurality ofselectively patterned light-absorbing layers. A semiconductor waferunder fabrication is provided. The wafer has a layer of photoresist atleast partially over the wafer. The semiconductor wafer is exposed tolight through the photomask. The photoresist is developed.

In accordance with a yet further aspect of the present invention, thereis provided a method for patterning a semiconductor device. A photomaskis provided. The photomask includes a substrate element. A first layerof a generally light-absorbing material is supported by the substrate.The first layer is over at least a first portion of the substrate. Abuffer layer is over the first layer. A second layer of a generallylight-absorbing material is supported over the buffer layer. The secondlayer is over a second portion of the substrate. The second portion is asubstrate of the first portion. A semiconductor wafer under fabricationis provided. The wafer has a layer of photoresist at least partiallyover the wafer. The semiconductor wafer is exposed to light through thephotomask. The photoresist is developed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 depicts a portion of a photomask blank manufactured in accordancewith the present invention, illustrated in vertical section;

FIG. 2 depicts the photomask blank of FIG. 1 during an intermediatemanufacturing step, also illustrated in vertical section;

FIG. 3 depicts a photomask manufactured from the photomask blank of FIG.1 including a multi-tiered photomask pattern, again illustrated invertical section;

FIG. 4 depicts an alternative embodiment of a photomask manufactured inaccordance with the present invention, again illustrated in verticalsection; and

FIG. 5 is a schematic representation of the photomasking process for themanufacture of a semiconductor device.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Described below is a photomask having multiple radiation-absorptionlevels. The photomask may be utilized with optical photolithographysystems and provide multiple absorption layers relative to alight-radiation surface. The photomask may be used with other types ofradiation, including infrared and ultraviolet, and other radiationmasking lithography operations wherein selective control of degree ofradiation transmission through a mask is of benefit to the operation.The photomark is a composite multi-layered structure, including aplurality of superimposed light-absorbing material layers and etch-stopmaterial layers. The multi-layered structure is manufactured byalternatively forming the layers over a photomask substrate, eachetch-stop material layer placed between two relatively light-absorbingmaterial layers.

Turning now to the drawings, FIG. 1 illustrates an exemplary embodimentof a photomask blank assembly 10 manufactured in accordance with thepresent invention. Photomask blank 10 includes three layers, 12, 14, and16 of a generally light-absorbing material. Placed between thelight-absorbing material layers 12, 14, and 16 are relatively thinnerlayers 20 and 22 of an etch-stop or buffer material. This buffermaterial is selected depending upon the etch process for the selectedlight-absorbing material. The term "light" as used herein is meant toinclude radiation having wavelengths in the visible spectrum, infrared,ultraviolet, deep ultraviolet regions (including G, H, and I-linewavelength radiation), and X-ray regions. Photomasks described hereinmay be utilized with these and other forms of radiation used in opticalphotolithography techniques or techniques wherein exposure may becontrolled through absorbing or blocking the radiation."Light-absorbing" as used herein is meant to define a material which,for the resist exposure wavelengths used in the selectedphotolithography technique, absorbs or reflects at least a portion ofthe light impinging the surface of the material. The term, thus,expressly includes materials which partially transmit light.

Light-absorbing layers 12, 14, and 16 are supported by an appropriatesupport surface 11, such as a quartz plate, soda-lime glass,borosilicate glass, or other suitable surface. Other suitable photomasksubstrate materials known in the art, such as sapphire, may also beused. Support surface 11 may be selected in accordance with propertiesthat have acceptable transmission thermal expansion and opticaltransmission characteristics for the specific application.

Selection of an appropriate light-absorbing material is generally basedon physical characteristics, optical properties, and etchingselectivity. A particularly advantageous material may exhibit relativelyhigh coefficients of hardness and toughness to reduce the possibility ofscratching or deformation of the photomask. The desired transmission andabsorption coefficient of the material may depend, among other factors,upon the range of the desired absorption levels of the finishedphotomask. Finally, the light-absorbing material may be selected ashaving a well-defined etching selectivity relative to the etch-stopmaterial for a satisfactory etchant.

While factors such as optical properties, chemical compatibility, andease of deposition may also affect the selection of the material for theinterlaced etch-stop layers, a principal factor is the ability of thematerial to act as an etching barrier during the etching of thelight-absorbing material layers. Likewise, the light-absorbing materialcan act as an etch barrier during removal or patterning of the etch-stoplayers.

An exemplary preferred light-absorbing/etch-stop material pair, used inthe embodiment illustrated in FIG. 1, is amorphous silicon and silicondioxide. Chromium and silicon dioxide could also be utilized as thelight absorbing/etch-stop material pair, potentially in combination withone or more layers of amorphous silicon. Other possible light-absorbingmaterials are titanium, titanium nitride, tungsten and molybdenum. Othersuitable material pairs or groupings may also be used.

To manufacture photomask 10, an appropriate photomask support surface 11is first provided. Support surface 11 is originally prepared by cuttingglass or quartz plates from large sheets. In a conventional manner, theplates are cleaned to remove chips and graded for flatness. The platesare further polished, cleaned, and inspected before the application ofthe first light-absorbing layer 12. Alternate layers of light-absorbingmaterial 12, 14, and 16 and etch-stop material 20 and 22 are then formedover the support surface 11. One method of deposition of an amorphoussilicon light-absorbing layer 12, 14, or 16 may be low pressure chemicalvapor deposition (LPCVD) conducted at temperatures generally below 580°C. The SiO₂ etch-stop layers 20 and 22 may be deposited using lowtemperature deposition of SiO₂, such as in an atmospheric pressure CVDreactor (APCVD) or in a plasma-enhanced CVD reactor (PECVD).

In most processes, it is important that the SiO₂ deposition occurgenerally below approximately 580° C. so as to preserve the amorphousstate of the deposited silicon layer(s). Although the thickness of thelayers may vary depending on the selected light spectrum and exposuretime, the thickness for the amorphous silicon layers 12, 14, and 16 ofthe present embodiment is 300-800A, and the thickness for the SiO₂layers 20 and 22 of the present embodiment is 100-500A.

FIG. 2 illustrates a step in the manufacture of photomask 10 whereinlayers 12, 20, and 14 have been formed successively over supportingsurface 11 using a deposition process or other suitable method. Thenumber of light-absorbing and etch-stop layers can be readily altered inalternative embodiments. Generally, for every n layers oflight-absorbing material, there will be n-1 layers of etch-stopmaterial. The number of layers is determined by the desired number ofabsorption levels. In general, n layers of silicon with n-1 interveninglayers of silicon oxide will provide for at least n+1 possibleabsorption or exposure levels. More absorption levels are possible (2n),if the intermediate etch-stop layer also comprises a suitablelight-absorbing material.

Together the layers 12, 14, 16, 20, and 22 form a compositemulti-layered structure. Upon formation of the last layer, the photomaskblank 10 can be patterned by removing portions 23 and 25 of one or morelayers along selected regions in response, for example, to the size ofthe features being defined. FIG. 3 illustrates a photomask 110manufactured from the photomask blank 10 shown in FIG. 1.

In the present embodiment, to pattern photomask blank 10 into photomask110, portions of one or more layers along selected regions aresuccessively and selectively removed by repeated etching steps. Thehigher layers, that is, the layers farthest from the support surface 11,can serve, at least partly, as masks for the patterning of the lowerlayers. Alternatively, the layer immediately below a layer which isbeing etched, acts as an etching barrier to prevent damage to underlyinglayers. The photomask 110 can be patterned to have a positive polarity(clear-field) or a negative polarity (dark-field). Dry etching is apreferred removal method, although wet etching or other suitable methodscan also be utilized. In the embodiment pictured in FIG. 3, the layers12, 14, and 16 of amorphous silicon may be removed, for example, usingplasma etching by an etchant including chlorine and/or brominecontaining gases, with or without diluent neutral gases. The siliconoxide layers 20 and 22 can be etched using a fluorine containing gas,such as CF₄, C₂ F₆, or C₃ F₈, and possibly hydrogen containing gas, suchas CHF₃, to aid etching selectivity.

The levels of absorption of different regions of photomask 110 can beprecisely controlled by controlling the removal of layers along thoseregions. In the depicted embodiment, with three layers 12, 14, and 16 ofamorphous silicon, photomask 110 can provide at least four differentlevels of exposure. All layers are removed along those regions wheremaximum exposure is desired (e.g., region D). Conversely, regions wherethe least exposure (and maximum photomask light-absorption) is desiredwould be those where all three layers 12, 14, and 16 are left intact(e.g., region A). Intermediate exposure levels can be accomplished byremoving one layer 16 (e.g., region B) or two layers 14 and 16 (e.g.,region C) of the amorphous silicon, along with none, one, or two of theintervening silicon oxide layers 20 and 22. Each layer oflight-absorbing material 12, 14, and 16 and an associated etch-stoplayer 20 and 22 can be selectively removed if desired in accordance withconventional resist and etching operations, each as might be implementedin forming a single level mask.

An important aspect of the selected embodiments described herein is theability to etch the light-absorbing (amorphous silicon) layers 12, 14,and 16 in a manner which essentially stops upon reaching the interveningetch-stop (silicon oxide) layers 20 and 22. Likewise, the etch-stoplayers 20 and 22 can be etched without undue attack on the underlyinglight-absorbing layers 12, 14, and 16.

The photomask 110 of FIGS. 1-3 relies exclusively upon selective etchingof multiple layers of uniform thickness to provide exposure andabsorption regulation. An alternative implementation, however, is tocontrol exposure through control of the thickness and the materialcomposition of one or more of the individual layers.

FIG. 4 illustrates an alternate embodiment of a photomask 210 havinglayers of different thicknesses and varying materials. Photomask 210includes a layer 216 of a first light-absorbing material and layers 214and 212 of a second light-absorbing material. Layer 212 is only half thethickness of layer 214, thus providing a different incremental level ofabsorption. By varying the thickness of the light-absorbing layers, theabsorption, and hence the exposure, can be accurately provided for anyrelative values. With more layers and different thicknesses almost anypractical distribution of optical absorption can be achieved. Therequired thickness of the different layers can be readily calculated byuse of well known optical formulae. For example, for a photomask havingfeatures as small as 0.25nm and for exposure using 365nm wavelengthradiation, a typical deposition thickness for the light-absorbing layers212, 214, and 216 will range between 100-800A, depending on theabsorption characteristics of the layer. At a thickness of 100-800A, thelight-absorption characteristics of an amorphous silicon layer rangefrom I/I₀ =0.5 to I/I₀ =0.004. For this example, typical silicon oxidelayers 20 and 22 will range from 100-500A, and offer a negligiblelight-absorbing range.

The present embodiment offers a photomask that contains regions whichhave varying levels of absorption and that, therefore, can produce animage that has received varying degrees of controlled exposure indifferent regions with a single exposure. Furthermore, by varying thenumber and the thickness of the layers 212, 214, and 216, the number ofdifferent levels of exposure that can be achieved simultaneously ispractically limitless.

Once the photomask 210 has been patterned and inspected, the imprintedpattern can be aligned or registered on a wafer of a semiconductordevice. FIG. 5 shows a flow diagram 300 showing the use of a photomaskin the fabrication of a semiconductor device. Step 310 includes firstproviding a photomask, such as photomask 210, manufactured in accordancewith the present invention. A semiconductor wafer under fabrication isthen provided in step 312.

A thin layer of a selected photoresist is applied in step 314 to theportion of the wafer wherein the photomask pattern is to be registered.Prior to this step, the wafer may be cleaned to remove contaminants andprimed, such as with hexamethyldisilazane (HMDS), to increasephotoresist adhesion. The photoresist is a light-sensitive material thatchanges its properties when exposed to light. Exemplary photoresists mayinclude positively acting etch-resistant organic polymers thatphotosolubilize in the presence of the selected form of light, such asan M-Cresolformaldehyde polymer. The photoresist may be selected torespond to a specific wavelength of light, differing exposing surfaces,resolution, polymerization or photosolubilization characteristics(negative or positive type photoresist), exposure speed, adhesion, orany specific needs of the specific application. In an exemplary method,the photoresist is applied to the surface to be processed in a liquidstate by a photoresist spinning process. The photoresist is spread overthe surface and dries to a thin film.

The photomask is precisely aligned over the surface to be treated instep 316. During step 318, the photomask and the underlying resistthrough the photomask are exposed to the selected form of light, such asultra-violet light in the present embodiment. The photomask pattern istransferred to the photoresist coating on the wafer surface by opticalprinting. Upon exposure to the selected light-source, the differentlight-absorbing layers of the photomask allow different levels ofexposure and, accordingly, different levels of polymerization orphotosolubilization of the photoresist. The time, intensity, and form ofradiation selected for exposure can be calibrated taking into accountthe desired pattern and the selected photoresist and photomaskmaterials.

Upon completion of the exposure step in step 320, the photoresist isdeveloped, such as by applying a solvent (developer) that does notaffect the underlying wafer, but which dissolves the unpolymerizedportions of the photoresist. The semiconductor wafer can then be etched,in step 322, using the remaining portions of the photoresist as anetch-resist mask. In step 324, the remaining photoresist is removed byprocesses known in the art, such as by an acetone soak.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the following appended claims.

What is claimed is:
 1. A semiconductor manufacture mask comprising:aplurality of radiation-absorbing layers comprising a first layer of afirst radiation-absorbing material and a second layer of a secondradiation-absorbing material, the first radiation-absorbing materialbeing different than the first radiation-absorbing material; and atleast one etch-stop layer, said etch stop layer disposed between saidfirst and second layers, wherein the plurality of radiation-absorbinglayers and the at least one etch-stop layer are patterned to define aplurality of radiation-absorbing regions having different radiationabsorption levels, wherein a first radiation-absorbing region comprisesmore than one of the plurality of radiation-absorbing layers and the atleast one etch-stop layer, and wherein a second radiation-absorbingregion comprises a lesser number of the plurality of radiation-absorbinglayers.
 2. The mask of claim 1, wherein one of said radiation-absorbinglayers has a different thickness than another of saidradiation-absorbing layers.
 3. A semiconductor manufacture mask havingmultiple light-absorption levels, comprising:a plurality oflight-absorbing layers, at least one of the light-absorbing layerscomprising amorphous silicon; and at least one etch-stop layer, saidetch-stop layer disposed between said light-absorbing layers, saidetch-stop layer comprising silicon dioxide, wherein the plurality oflight-absorbing layers and the at least one etch-stop layer arepatterned to define a plurality of light-absorbing regions havingdifferent light absorption levels, wherein a first light-absorbingregion comprises more than one of the plurality of light-absorbinglayers and the at least one etch-stop layer, and wherein a secondlight-absorbing region comprises a lesser number of the plurality oflight-absorbing layers.
 4. The mask of claim 3, further comprising anunderlying substrate.
 5. The mask of claim 4, wherein said substratecomprises a quartz photomask substrate.
 6. A method for manufacturing aphotomask having multiple light-absorbing levels, the method comprisingthe steps of:providing a photomask substrate; and forming a plurality oflayers of a light-absorbing material and a plurality of layers of anetch-stop material in an alternating fashion on said photomasksubstrate, the light-absorbing material and the etch-stop material beingselectively etchable in relation to each other, wherein the plurality oflayers of light-absorbing material comprise a first, second, and a thirdlayers of light-absorbing material, the first and third layers oflight-absorbing material comprising a first material, and the secondlayer of light-absorbing material comprising a second material differentthan the first material.
 7. The method of claim 6, further comprisingthe step of removing at least a portion of at least one of said layersusing an etching process.
 8. The method of claim 7, wherein said step ofremoving comprises the step of selectively etching a portion of one ofsaid layers of light-absorbing material using one of said layers ofetch-stop material as an etching barrier.
 9. The method of claim 7,wherein said step of removing comprises the step of selectively etchinga portion of one of said layers of etch-stop material using one of saidlayers of light-absorbing material as an etching barrier.
 10. The methodof claim 6, wherein said etch-stop material comprises silicon dioxide.11. The method of claim 6, wherein said first and third layers oflight-absorbing material comprise amorphous silicon.
 12. The method ofclaim 6, wherein one of said layers of light-absorbing material has adifferent thickness than another of said layers of light-absorbingmaterial.
 13. A method for manufacturing a photomask having multiplelight-absorbing levels, the method comprising the steps of:providing aphotomask substrate; depositing a first layer of a generallylight-absorbing material onto said substrate, said first layercomprising chromium; forming a second layer of a buffer material ontosaid first layer, said second layer of a buffer material selectivelyetchable relative to said first layer, said second layer comprisingsilicon dioxide; and depositing a third layer of a generallylight-absorbing material, said third layer of a material selectivelyetchable relative to said second layer.
 14. The method of claim 13,further comprising the step of removing at least a portion of at leastone of said layers using an etching process.
 15. The method of claim 14,wherein said step of removing comprises the step of selectively etchinga portion of said third layers of light-absorbing material using saidsecond layer of buffer material as an etching barrier.
 16. The method ofclaim 14, wherein said step of removing comprises the step ofselectively etching a portion of said second layer of buffer materialusing said first layer of light-absorbing material as an etching barrierand at least partly using said third layer as an etching mask.
 17. Themethod of claim 13, wherein said third layer comprises one of amorphoussilicon, chromium, titanium, titanium nitride, tungsten, and molybdenum.18. The method of claim 13, wherein one of said layers oflight-absorbing material has a different thickness than other of saidlayers of light-absorbing material.
 19. A method of manufacturing aphotomask, comprising the acts of:(a) providing a substrate; (b)disposing a first layer of light-absorbing material on the substrate,the first layer of light-absorbing material having a first thickness;(c) disposing a first layer of etch-stop material on the first layer oflight-absorbing material; (d) disposing a second layer oflight-absorbing material on the first layer of etch-stop material, thesecond layer of light-absorbing material having a second thicknessdifferent than the first thickness; (e) disposing a second layer ofetch-stop material on the second layer of light-absorbing material; and(f) disposing a third layer of light-absorbing material on the secondlayer of etch-stop material.
 20. The method, as set forth in claim 19,comprising the act of:forming a pattern window through at least thethird layer of light-absorbing material and the second layer ofetch-stop material.
 21. The method, as set forth in claim 20, comprisingthe act of:forming a pattern window through at least the second layer oflight-absorbing material and the first layer of etch-stop material. 22.The method, as set forth in claim 21, comprising the act of:forming apattern window through the first layer of light-absorbing material. 23.The method, as set forth in claim 19, wherein the first layer oflight-absorbing material is thinner than the second layer oflight-absorbing material.
 24. The method, as set forth in claim 19,wherein the first, second, and third layers of light-absorbing materialcomprise a same material.
 25. The method, as set forth in claim 24,wherein the same material comprises one of amorphous silicon, chromium,titanium, titanium nitride, tungsten, and molybdenum.
 26. The method, asset forth in claim 19, wherein the first and third layers oflight-absorbing material comprise a first material and wherein thesecond layer of light-absorbing material comprises a second materialdifferent than the first material.
 27. The method, as set forth in claim19, wherein the first and second layer of etch-stop material comprise asame material.
 28. The method, as set forth in claim 27, wherein thesame material comprises silicon dioxide.
 29. A method of manufacturing aphotomask, comprising the acts of:(a) providing a substrate; (b)disposing N layers of light-absorbing material over the substrate, whereN is an integer greater than 2 and the light-absorbing materialcomprises amorphous silicon; and (c) disposing N-1 layers of etch-stopmaterial over the substrate between the respective N layers oflight-absorbing material, where the etch-stop material comprises silicondioxide.
 30. The method, as set forth in claim 29, comprising the actof:forming a pattern window through at least one of the N layers oflight-absorbing material and at least one of the N-1 layers of etch-stopmaterial.
 31. The method, as set forth in claim 30, comprising the actof:forming a pattern window through at least two of the N layers oflight-absorbing material and at least two of the N-1 layers of etch-stopmaterial.
 32. The method, as set forth in claim 31, comprising the actof:forming a pattern window through all of the N layers oflight-absorbing material and all of the N-1 layers of etch-stopmaterial.
 33. The method, as set forth in claim 29, wherein at least oneof the N layers of light-absorbing material has a thickness differentthan the other of the N layers of light-absorbing material.
 34. Aphotomask comprising:a substrate; a first layer of light-absorbingmaterial disposed over the substrate, the first layer of light-absorbingmaterial comprising amorphous silicon and having a first thickness; afirst layer of etch-stop material disposed over the first layer oflight-absorbing material, the first layer of etch-stop materialcomprising silicon dioxide; a second layer of light-absorbing materialdisposed over the first layer of etch-stop material, the second layerhaving a second thickness different than the first thickness; a secondlayer of etch-stop material disposed over the second layer oflight-absorbing material; a third layer of light-absorbing materialdisposed over the second layer of etch-stop material; a first patternwindow formed through at least the third layer of light-absorbingmaterial and the second layer of etch-stop material; a second patternwindow through at least the second layer of light-absorbing material andthe first layer of etch-stop material; and a third pattern windowthrough the first layer of light-absorbing material.
 35. The photomask,as set forth in claim 34, wherein the first layer of light-absorbingmaterial is thinner than the second layer of light-absorbing materialand thinner than the third layer of light-absorbing material.
 36. Thephotomask, as set forth in claim 34, wherein the second layer oflight-absorbing material comprises amorphous silicon.
 37. The photomask,as set forth in claim 34, wherein the second layer of light-absorbingmaterial comprises one of amorphous silicon, chromium, titanium,titanium nitride, tungsten, and molybdenum.
 38. The photomask, as setforth in claim 34, wherein the third layer of light-absorbing materialcomprises amorphous silicon and wherein the second layer oflight-absorbing material comprises a second material different thanamorphous silicon.
 39. The photomask, as set forth in claim 34, whereinthe second layer of etch-stop material comprises silicon dioxide.
 40. Aphotomask, comprising:a substrate; N layers of light-absorbing materialdisposed over the substrate, where N is an integer greater than 2, atleast one of the N layers of light-absorbing material comprisingamorphous silicon; N-1 layers of etch-stop material disposed over thesubstrate between the respective N layers of light-absorbing material,at least one of the N-1 layers of etch-stop material comprising silicondioxide; a first pattern window formed through at least one of the Nlayers of light-absorbing material and at least one of the N-1 layers ofetch-stop material; a second pattern window formed through at least twoof the N layers of light-absorbing material and at least two of the N-1layers of etch-stop material; and a third pattern window through all ofthe N layers of light-absorbing material and all of the N-1 layers ofetch-stop material.
 41. The photomask, as set forth in claim 40, whereinat least one of the N layers of light-absorbing material has a thicknessdifferent than the other of the N layers of light-absorbing material.42. The photomask, as set forth in claim 40, wherein the other of the Nlayers of light-absorbing material comprise a same material.
 43. Thephotomask, as set forth in claim 42, wherein the same material comprisesone of amorphous silicon, chromium, titanium, titanium nitride,tungsten, and molybdenum.
 44. The photomask, as set forth in claim 40,wherein the other of the N-1 layers of etch-stop material comprise asame material.
 45. The photomask, as set forth in claim 44, wherein thesame material comprises silicon dioxide.